fix(git): keep worktree origin base without inheriting master upstream

[SDSLeon]

Summary

• New worktrees now default to the origin/tracking ref (T3-style). Picking master in the dropdown stays on origin/master. Worktree + changes and Branch stay on the local checkout.
• Creating a worktree from origin/master no longer sets that ref as the new branch's git upstream. Git's default autoSetupMerge was making poracode/… track origin/master, which showed a false "behind" badge and broke git push (upstream branch does not match).
• Existing worktrees that already inherited the fork base as upstream are repaired on the next git status (branch --unset-upstream).

Test plan

• pnpm exec vitest run on BranchSelector, ThreadDraftView, git.test, and status parser tests
• Switch a draft to Worktree: trigger shows origin/<branch>; selecting the local name keeps origin
• Switch to Worktree + changes: trigger stays on the local branch and still offers to copy uncommitted files
• Create a new worktree from origin/master: git status -sb is poracode/… with no ...origin/master
• git push -u origin HEAD publishes origin/poracode/… without the name-mismatch error